Vehicle tires support wheel axle load on a tread area in contact with a road surface. The tire contact area multiplied by inflation pressure will be equal to the wheel axle load.
Coefficient of friction between tread and road surface multiplied by the axle load is the maximum force, parallel to the road surface, that can be applied to the tire contact area by the wheels to stop, accelerate, corner or maintain speed on a grade without tire slippage at the contact between the road and tire. As an example, if a tire tread coefficient of friction is 0.30, and an axle load is 1,000 pounds, the maximum friction force between the tire and road surface before tire slippage will be 300 pounds. If a braking load or an acceleration load, greater than 300 pounds at the tread/road interface is developed, wheel rotation will decrease or increase respectively and when the slippage occurs to some degree, a loss of control may occur.
Almost all automobiles have brakes and engine capacities sufficient to generate loads that exceed tire friction traction on dry concrete, asphalt, gravel, or dirt. This capacity introduces a responsibility for vehicle operators to use restraint from applying full throttle when accelerating and using maximum braking and steering to maintain safe operation and reasonable service life of tires and vehicle components.
The coefficient of friction between tires and wet pavement is moderately reduced from dry conditions and requires drivers to use longer stopping distances and lower maximum acceleration values. Driving on wet level, sloped, or curved roads with conventional tires has been found to be manageable by drivers notwithstanding that wet brakes and potential for hydroplaning introduce a need for caution.
The coefficient of friction between tire tread and ice is so low, however, control of a vehicle on ice covered roads under most conditions at moderate speed is precluded unless surface friction has been increased by sand or chemicals, or unless the vehicle's tires have been fitted with chains, or studs to develop reactive forces.
Loss of control on ice is typically evidenced by spinning wheels when initiating motion, locking of wheels when braking, and lack of steering response (e.g. “understeering”) due to insufficient friction between the vehicle's tires and road surface for vehicle traction.
Tire chains and tire studs function by impressing a fixed-shape chain or stud component into ice by tire tread to develop tractive force which is limited by shear values of ice and geometry of components.
Some known designs for tires with retractable studs rely on perpetual maintenance of air pressure in order to maintain the associated stud in a deployed position. Additionally, some systems use, and thus can deplete, air within a tire in order to cause movement of a stud, for example, between a retracted and a deployed position.